Concern has been expressed about the possibility that strenuous exercise in young individuals (e.g., marathon running by young children) might have deleterious effects on bone growth. We plan to study the effect of a carefully controlled high intensity exercise program on the growth plates of the long bones of White Leghorn chickens. These animals were chosen because they are bipedal, can easily be trained to run and their growth plates are, in all essential aspects, similar to those of mammals. The chicks will be exercised daily, starting at four weeks of age, by running on a treadmill at a speed and grade corresponding to 75-80% of their maximal oxygen consumption, sacrificed at 8, 12, 16 and 20 weeks of age and compared to pair-weighed controls. Some animals will be exercised up to 28 weeks of age for studies on menisci and articular cartilage. We will assess the effect of the exercise program on the weight, length and cross sectional areas of the long bones and on the histological and ultrastructural properties of the tibial growth plates. The effect on cartilage matrix metabolism at the different ages and after different durations of exercise will be assessed by studying the composition, properties and biosynthesis of proteoglycans and the rates of synthesis and degradation of collagen. We will also study the effect of prolonged high intensity exercise on the histological properties, collagen and proteoglycan content and proteoglycan characteristics of articular cartilage and menisci of animals at 28 weeks of age when bone growth has ceased. This histological and biochemical study of the effect of a controlled high intensity exercise program on the growth plates will furnish data indicating whether or not such a program may have a negative effect on bone growth in rapidly growing animals and should furnish indications as to whether or not there is scientific justification for the growing concern about participation of young children in long distance running programs.